The present invention relates generally to flexible optical circuits and, more particularly, to flexible optical circuits designed to provide strain relief for the optical fibers.
Flexible optical circuits are utilized in a wide variety of applications in which fiber management is desirable. For example, flexible optical circuits are commonly utilized as optical backplanes to interconnect a number of printed circuit boards or the like. Similarly, flexible optical circuits can serve as ribbons of optical fibers in order to route the optical fibers in an organized fashion.
Regardless of the application, a flexible optical circuit is commonly formed of a plastic substrate, typically formed of a polyimide or similar types of engineering thermoplastic materials, such as polyetherimide or polybutylene terphthalate. Most commonly, however, the substrate is formed of Kapton(trademark) polyimide. The substrate is coated with an adhesive, such as a silicone adhesive, and a plurality of optical fibers are placed upon the adhesive-coated substrate. In particular, the optical fibers are placed in a predetermined pattern upon the substrate in order to appropriately route the optical fibers. The flexible optical circuit is then completed by placing another layer over the optical fibers. For example, a flexible optical circuit can include another layer of the plastic material that forms a substrate in order to effectively sandwich the optical fibers between the layers of plastic. By way of example, the flexible optical circuit can include a layer formed of Kapton(trademark) polyimide that overlies the optical fibers and is adhered to the substrate. Alternatively, a conformal coating can be applied so as to cover the substrate and the optical fibers adhered to the substrate. For example, a conformal coating of silicone can be sprayed on the substrate in order to cover the optical fibers as well as other portions of the substrate.
In designing a flexible optical circuit, it is desirable for the flexible optical circuit and, more particularly, for the components that form the flexible optical circuit, to be flame retardant. Additionally, a flexible optical circuit preferably has good environmental resistance properties. In this regard, the flexible optical circuit preferably maintains approximately the same generally small level of attenuation for signals transmitted via the optical fibers as the temperature and humidity to which the flexible optical circuit is subjected are varied within a predetermined range of temperatures and humidities. Still further, the flexible optical circuit preferably has good handling characteristics. In other words, the flexible optical circuit is preferably relatively flexible to facilitate routing of the optical fibers. As such, the flexible optical circuit must be capable of being readily bent or otherwise flexed.
Unfortunately, the substrate of most flexible optical circuits is much less flexible than the optical fibers. In other words, the substrate of most flexible optical circuits is relatively stiff or inflexible relative to the optical fibers. This relative inflexibility is compounded in instances in which the flexible optical circuit includes a second layer of plastic, such as a second layer formed of Kapton(trademark) polyimide, that covers the optical fibers and other portions of the substrate. Accordingly, conventional flexible optical circuits disadvantageously subject the optical fibers to stress.
Typically, the stress is concentrated upon the optical fibers at one or more points depending upon the configuration of the flexible optical circuit. For example, most flexible optical circuits are designed such that the optical fibers extend beyond the edge of the substrate. As such, the optical fibers are subjected to stress along the edge of the substrate. Additionally, fiber optic connectors are commonly mounted upon respective optical fibers of a flexible optical circuit. In these instances, the substrate upon which the optical fibers are mounted typically extends into the connector boot and into the rearward end of the spring push element of the fiber optic connector. Nevertheless, the optical fibers are typically subjected to stress at the point beyond the edge of the substrate at which the fiber optic connector is mounted to the optical fiber. In embodiments in which the flexible optical circuit includes a second layer of plastic, such as a second layer of Kapton(trademark) polyimide, that covers the optical fibers and the substrate, the second layer of plastic may delaminate and peel back from the substrate as the flexible optical circuit is bent or otherwise flexed. In these instances, stress is also concentrated on the optical fibers at the point at which the second layer of plastic becomes delaminated from the substrate. In each of these instances, the points along the optical fiber at which the stress is concentrated will disadvantageously increase the attenuation of the optical signals transmitted via the optical fibers.
In order to protect the optical fibers from the concentrations of stress, some flexible optical circuits have included shrink tubing and/or strain relief boots through which optical fibers extend. By positioning the shrink tubing or the strain relief boot upon that segment of the optical fiber at which the stress is concentrated, the optical fiber can be at least partially shielded from the stress such that the signals propagating along the optical fibers are not attenuated to the same degree. For example, the shrink tubing or strain relief boot may be placed upon an optical fiber proximate the edge of the substrate in order to protect the optical fiber from the concentration of stress that typically occurs at the edge of the substrate.
In addition to increasing the cost of a flexible optical circuit, strain relief boots and shrink tubing create other difficulties. In this regard, strain relief boots and shrink tubing are typically rather bulky, and/or inflexible relative to the remainder of the flexible optical circuit. As such, the resulting flexible optical circuit is typically heavier and somewhat more difficult to handle than conventional flexible optical circuits that do not include either strain relief boots or shrink tubing. Additionally, it is generally more difficult and laborious to fabricate flexible optical circuits that include strain relief boots and/or shrink tubing, thereby increasing the time required for fabrication and, in many instances, the cost of the resulting flexible optical circuit.
Even in instances in which segments of the optical fibers are protected from concentrations of stress by strain relief boots and/or shrink tubing, the majority of the length of the optical fibers is not protected by strain relief boots and/or shrink tubing. As such, these other unprotected segments of the optical fibers are susceptible to damage and therefore increased attenuation as a result of inadvertent contact with the optical fibers. For example, flexible optical circuits are typically deployed in electronics cabinets or other closures that also house a variety of other components, typically formed of metal or hard plastic. As such, these other components may inadvertently contact the flexible optical circuit during installation or subsequently during the repair, thereby damaging the optical fibers if the components contact those segments of the optical fibers that are not protected by a strain relief boot or shrink tubing. Accordingly, most conventional flexible optical circuits disadvantageously have a relatively small crush resistance.
An improved flexible optical circuit is therefore provided that includes at least one layer formed of a foam material in order to provide strain relief for the optical fibers and to improve the crush resistance of the flexible optical circuit. Accordingly, the flexible optical circuit need not include strain relief boots and/or shrink tubing such that the flexible optical circuit is simpler to manufacture and generally less expensive than conventional flexible optical circuits having strain relief boots and/or shrink tubing.
A flexible optical circuit includes a substrate and at least one optical fiber disposed upon the substrate. In one advantageous embodiment, the substrate is formed of a foam material, such as a silicone or polyurethane foam. As such, the foam substrate provides both crush resistance and strain relief for the optical fibers. Preferably, the foam material that forms the substrate includes a non-porous surface upon which the optical fibers are mounted. Additionally, the foam material that forms the substrate is preferably flame retardant. Typically, the flexible optical circuit also includes an adhesive, such as a silicone adhesive, disposed upon at least a portion of the substrate for attaching the optical fibers to the substrate. The flexible optical circuit can also include a conformal coating disposed upon the substrate and overlying the optical fibers.
In addition to the substrate and the optical fibers disposed upon the substrate, the flexible optical circuit of one advantageous embodiment also includes a protective layer disposed upon at least a portion of the substrate so as to overlie at least a segment of at least one optical fiber. According to this embodiment, at least one of the substrate and the protective layer is formed of a foam material to provide strain relief for the optical fibers. In this regard, the substrate may be formed of the foam material as described in conjunction with the foregoing embodiment. Alternatively, the protective layer may be formed of the foam material, irrespective of whether the substrate is also formed of a foam material or is formed of another material, such as a polyimide or other plastic material. In embodiments in which the protective layer is formed of a foam material, the protective layer also advantageously provides strain relief for the optical fiber and, at least some, crush resistance for the optical fibers.
In embodiments in which the protective layer is formed of a foam material, the foam material also preferably includes a non-porous surface facing the optical fibers. In addition, the foam material that forms the protective layer of these embodiments is preferably flame retardant and is typically formed of either a silicone or polyurethane foam. In embodiments in which the protective layer is formed of a foam material, the protective layer may cover the entire substrate including all segments of the optical fibers. Alternatively, the protective layer may be designed to cover those segments of the optical fibers subjected to the largest concentrations of stress, while leaving other segments of the optical fibers exposed. In this regard, the protective layer of foam material is preferably disposed proximate the edges of the substrate to protect the optical fibers from the concentrations of stress that otherwise occur at the edge of the substrate. In this regard, the substrate can include a main section and at least one tab extending outwardly therefrom. In this embodiment, the protective layer of foam material is preferably disposed upon at least a portion of the at least one tab. Additionally, the flexible optical circuit may include at least one fiber optic connector mounted upon a respective optical fiber. In this embodiment, the protective layer of foam material is preferably disposed proximate the fiber optic connector in order to protect the optical fiber from the concentration of stress that otherwise would occur at the fiber optic connector. Since the protective layer generally does not cover the entire substrate, the flexible optical circuit can also include a conformal coating disposed upon the substrate and overlying the optical fibers. The protective layer of foam material can then be disposed upon portions of the conformal coating, such as those portions at which the stress is concentrated upon the optical fibers.
In each of these embodiments, the improved flexible optical circuit provides strain relief for the optical fibers as a result of the inclusion of a layer of foam material. As a result of the strain relief, the improved flexible optical circuit can transmit signals with lower levels of attenuation. Moreover, the improved flexible optical circuit provides improved crush resistance, thereby protecting the flexible optical circuit and, in particular, the optical fibers from physical damage as a result of contact with other components within an electronics cabinet, closure or the like. Additionally, the improved flexible optical circuits of the present invention and, in particular, those embodiments of the improved flexible optical circuits that include a substrate formed of a foam material are quite flexible, thereby reducing the concentration of stress upon the optical fibers and improving the handling characteristics of the flexible optical circuit so as to facilitate installation, repair and the like of the improved flexible optical circuit. Further, the improved flexible optical circuits of the present invention can be efficiently fabricated, thereby reducing the time required for manufacture and the costs of manufacture relative to the fabrication of conventional flexible optical circuits that include shrink tubing and/or strain relief boots.